In a hair modification treatment of changing the hair shape close to a wave form or a straight form, a first agent for hair modification including a reducing agent is usable, and cleavage of cystine bonds in hair caused by that reducing agent enables modification of the hair shape. Furthermore, in a process for hair modification treatment, there are occasions in which the hair is heated, and there is known a method of heating hair that is coated with a first agent for hair modification to a temperature of 60° C. or lower, for the purpose of promoting the penetration of the first agent for hair modification into the hair. In addition to that, there are also occasions in which the hair is brought into contact with a heating element at a higher temperature, and there is also known a method of washing away the first agent for hair modification from the hair, and then heating the hair with a heating element such as a rod at a temperature of 70° C. or higher, or a hair iron at a temperature of 100° C. or higher.
Heating as described above may intensify the damage of hair, and as the heating temperature is higher, the possibility of hair being damaged is increased. Since the feeling of hair to the touch is deteriorated as the hair is damaged, suggestions have been made to suppress the deterioration. For example, JP 2004-26770 A discloses a hair straightening treatment carried out by using a hair iron, which is a kind of hair modification treatment, and also discloses that gluconic acid and trehalose are incorporated into a first agent for hair straightening so as to promote enhancement of a moist feeling and softness of hair when the hair is subjected to the aforementioned treatment.